1. Field of the Invention
The present invention relates to an oscillator controller incorporating a voltage-controlled oscillator that outputs an oscillation signal at a desired oscillation frequency.
2. Background Art
Recently, for example, in the field of mobile communication, the market of which is rapidly growing, there is an increasing demand that a voltage-controlled oscillator (VCO) be mounted on each mobile communication terminal (see Japanese Patent Laid-Open No. 2006-504352, for example).
The voltage-controlled oscillator is capable of changing the oscillation frequency according to an oscillation frequency controlling voltage.
However, recently, the circuit operating voltage has been lowered, and accordingly, the available range of voltage variation that assures an adequate performance has become narrower. In addition, a wider band of frequencies has come into use. A conventional VCO increases the amount of frequency change per unit voltage. As a result, the sensitivity to the noise on the controlling voltage increases, causing deteriorations of characteristics, such as phase noise and spur.
Thus, in order to reduce the phase noise while widening the frequency band at low voltage, voltage-controlled oscillators have become widely used which have another external control terminal in addition to that for frequency control and can selectively change the oscillation frequency band.
By the way, there is a typical LC resonator that can selectively change the capacitance using a switch to select the oscillation band. If such an LC resonator is used, a sufficient oscillation frequency band can be assured while reducing the frequency change or oscillation frequency controlling voltage change.
However, since the frequency change or oscillation frequency controlling voltage change is reduced, the range of frequency change is reduced when the switch for frequency adjustment is fixed.
Therefore, when using such a circuit, the state of the switch has to be appropriately changed according to the oscillation frequency.
In addition, there is a conventional voltage-controlled oscillator that raises the frequency as the controlling voltage rises. Such a voltage-controlled oscillator compares a reference voltage and an oscillation frequency controlling voltage to make the oscillation frequency controlling voltage fall within an ideal range.
If the oscillation frequency controlling voltage is higher than the reference voltage, a capacitor is opened to raise the frequency to lower the oscillation frequency controlling voltage. If the oscillation frequency controlling voltage is lower than the reference voltage, the capacitor is closed to lower the frequency to lower the oscillation frequency controlling voltage.
The oscillation frequency adjustment method of the conventional voltage-controlled oscillator has a problem that the time required for the adjustment depends on the characteristics of a phase locked loop (PLL).
Specifically, since the value of the oscillation frequency controlling voltage is used as it is, when the switch circuit for controlling the composite capacitance is set at a certain value, the time required for determining the result of comparison between the reference voltage and the oscillation frequency controlling voltage depends on the constants of circuits constituting the PLL.
Furthermore, there is another conventional voltage-controlled oscillator that, when setting the capacitance of the VCO, stops a phase frequency detector, compares the reference voltage and the oscillation frequency controlling voltage in synchronization with the reference signal, and drives a charge pump based on the result of the comparison to make the oscillation frequency controlling voltage nearly equal to the reference voltage (reference voltage≦oscillation frequency controlling voltage).
In addition, the output of a prescaler, rather than the output of a frequency divider, is counted. At this time, the frequency of the VCO can be accurately determined by checking the control signal for the prescaler at the same time.
With such a configuration, the comparison can be carried out within a shorter time than the case where the output of the frequency divider is used for the comparison. And the charge pump is shared, so that the deteriorations of the characteristics due to addition of an extra circuit, such as a switch, described above can be avoided.
However, in the case where the output of the prescaler is compared with the reference signal, the output frequency of the prescaler and the output frequency of the reference signal naturally differ from each other by an amount which is counted by a pulse swallow counter.
In particular, in the case of the fractional-N PLL using a sigma delta modulator, settings of the pulse swallow counter are various.
Therefore, in order to provide an accurate comparison coefficient, calculations have to be carried out again from setting values of the PLL. As a result, the circuit size increases, and a comparison error occurs depending on the selection of the frequency used as a reference frequency.